According to 2010 statistics, if you combine the London airports they are the busiest set of airports in the world (by pax - I haven't checked per movements):
World's busiest city airport systems by passenger traffic - Wikipedia, the free encyclopedia
Passenger numbers don't always equate to number of aircraft in the sky (Heathrow being a prime example..not even in the top 10 for aircraft movements worldwide but in the top 3 passenger list).
Whilst I agree the airspace over the SE is congested I agree with other posters that it will take a LOT to change it as there are too many conflicting interests.
It seems to get a lot of people's backs up on PPRUNE..but the US does seem to manage congested airspace a little bit easier than the Brits (other issues aside). Los Angeles has already been mentioned with Van Nuys and LAX operation side by side....probably close to 900 000 movements a year between the two. New York should be the prime example. JFK, Newark, La Guardia and Teterboro....over one million aircraft a year....operating within approximately 25 square miles of each other and all serving a different purpose (international, domestic, combination, GA etc).
Why can't we use that example to help over the South East...especially given that airports like Stansted and Luton are quieter than LGA and EWR and are relatively further away from Heathrow than the New York airports are from each other?
Please don't mention "safety concerns" because it seems safe enough for most major airline carriers...including our flagship British one.